Cracking furnace with burners engaging mirror image coiled tubes

ABSTRACT

A cracking furnace for carrying out chemical reactions wherein the furnace is a closed chamber having a top bottom and at least two relatively smooth vertical parallel walls. A plurality of vertically extending hollow tubes for carrying a medium therethrough are disposed in the chamber, an odd number of the tubes forming a coil with all of the tubes in one coil being joined to adjacent tubes in the same coil. The coils include inlet and outlet openings, the inlet openings being on one side of the chamber and the outlet openings on the other. At least two coils are adjacent in such a manner that one is the mirror image of the other. Burner means operatively engages the inlet openings and a combustion gas outlet in the chamber is in communication with the outlet openings.

ite States Patent [72] Inventor Wilhelmus R. Dorresteyn 3,407,789

Amsterdam, Netherlands [211 App]. No. 847,285 [22] Filed Aug. 4, 1969 [45] Patented Dec. 28, 1971 [73] Assignee Shell Oil Company New York, N.Y. [32] Priority Dec.2,1968 3 3 I Netherlands I 3 1 6817224 [54] CRACKING FURNACE WITH BURNERS ENGAGING MIRROR IMAGE COILED TUBES 12 Claims, 4 Drawing Figs.

[51] Int. Cl Cl0g 9/20 [50] Field ofSeareh.... 196/110,

[5 6] References Cited UNITED STATES PATENTS 3,215,502 11/1965 Korwin l96/11OX 1 amass 10/1968 Hallee et al Primary ExaminerNorman Yudkolf Assistant Examiner- David Edwards Attorneys Louis J. Bovasso and J. H. McCarthy ABSTRACT: A cracking furnace for carrying out chemical reactions wherein the furnace is a closed chamber having a top bottom and at least two relatively smooth vertical parallel walls. A plurality of vertically extending hollow tubes for carrying a medium therethrough are disposed in the chamber, an odd number of the tubes forming a coil with all of the tubes in one coil being joined to adjacent tubes in the same coil. The coils include inlet and outlet openings, the inlet openings being on one side of the chamber and the outlet openings on the other. At least two coils are adjacent in such a manner that one is the mirror image of the other. Burner means operatively engages the inlet openings and a combustion gas outlet in the chamber is in communication with the outlet openings.

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INVENTOR:

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WILLIAM R. DORRESTEYN av fwggo ms ATTORNEY CRACKING FURNACE WITH BURNERS ENGAGING MIRROR IMAGE COILED TUBES BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to a furnace with vertical tubes for supplying heat to a medium that can he passes through the tubes, suitable for carrying out chemical reactions such as pyrolysis.

2. Description of the Prior Art Furnaces of this type are employed for thecracking of hydrocarbons, for instance for the preparation of ethene from naphtha. In order to promote the desired chemical reactions as far as possible, the incoming hydrocarbon mixture should rapidly be brought to a very high temperature, for instance 850 C., and be kept at least at that temperature for a short time. A furnace with vertical tubes is very suitable for such high temperatures, because sagging of the tubes cannot then occur and expansion of tubes can more easily be accommodated constructionally. The vertical tubes are located in a chamber which in its simplest form is bounded by smooth, vertical walls, a floor and a roof which chamber also contains the burners. Naturally, this chamber is provided with the required lines and ducts for the supply of feed, fuel and air and for the discharge of product and combustion gases. In this chamber the transfer of heat to the tubes takes place mainly by radiation. It is common practice for the heat content of the combustion gases leaving the chamber to be utilized as far as possible by preheating the feed by these gases in a separate convection chamber. As a rule this convection chamber is located very close to the outlet for the discharge of the combustion gases.

It is common practice to use a large number of burners that are distributed over two or more vertical sidewalls of the chamber. The number of burners ranges from 50 to 100. Each of these burners has a comparatively small capacity.

A furnace fired in this way has several disadvantages. The use of residual oil as fuel is not possible. In view also of the desired controllability of the burners, gas is fired, which is comparatively expensive. In addition, the large number of burners results in a complicated system of fuel supply and control valves. Furthermore there is a steep tube temperature gradient along the route from the feed inlet to the product outlet.

SUMMARY OF THE INVENTION It is an object of this invention to provide an improved cracking furnace for supplying heat to a medium that can be passed through vertical tubes within the furnace.

The invention therefore relates to a furnace provided with vertical tubes-which are present in a chamber of which at least two walls are relatively smooth, vertical and parallel to each otherfor the supply of heat to a medium that can be passed through the tubes, suitable for carrying out chemical reactions such a-pyrolysis, in which furnace:

a. an odd number of tubes is combined into a coil by joining adjacent tubes in such a way that connections at the bottom ends of the tubes alternate with connections at the top ends of the tubes,

b. an even number of coils is present,

c. the inlet openings of all the coils are located on the same side of the furnaceat the bottom or at the top,

d. the outlet openings of all the coils are likewise located on the same side of the furnaceat the top or at the bottom-opposite the inlet openings,

e. at least two coils are placed next to each other, so that one of two adjacent coils is the mirror image of the other, the tubes being located in one plane, parallel to the aforementioned vertical walls of the chamber, in such a way that always two inlet openings are adjacent,

f. vertical burners giving short flames are employed, which burners are located on both sides of the coils near the inlet openings,

g. one or more outlets for the discharge of combustion gases is (are) located on the side of the furnace where the outlet openings of the coils are located.

The furnace according to the invention has no burners that are mounted in the sidewalls. The burners are located in the bottom or in the roof. near the inlet openings of the coils. The number of burners is small and each burner has a comparatively large capacity, the class of burners with such a capacity comprising burners suitable for firing residual oil. By the composition and arrangement of the coils and the burners, it is ensured that the medium being passed through the tubes immediately after having entered the furnace is close to the burners and that the product leaving the tubes is as far removed from the burners as possible. The difference in temperature between the medium and the hot combustion gases is largest at the point of entrance, so that the maximum amount of heat can be transferred to the medium, mainly by radiation. One wishes the high temperature required for the desired conversions, for instance the cracking of hydrocarbons, to be reached within a short time. The net heat flux, that is the amount of heat absorbed by the medium flowing through the tubes, then should be distributed unevenly over the length of each coil in the direction from the inlet opening to the outlet opening. For, after the desired temperature has been reached, no heat is required any more for effecting a further rise in temperature, though heat continues to be required for the conversions on account of the endothermic character of the reactions. Also, according as the contents of the coil approach the outlet opening, less heat is required for conversions, because most of the conversions will by then have been effected. In a furnace according to the invention a coil consists of at least three tubes. The second and following tubes, counting from the tube with the inlet opening, are farther removed from the burners, as a result of which the heat supply is decreased. It may be desirable that the distances between adjacent tubes of one bank be unequal, that is, should increase along the route from the tube with the outlet opening to the tube with the inlet opening. Then the largest distance is between the first and the second tube, counting from the tube with the inlet opening, as a result of which that section of the second pipe which again comes near the burner is prevented from being heated to too high a temperature. The distances between the first and the second tube may, for instance, be three times the distance between the second and the third. Thus, in a furnace according to the invention the net heat flux is unevenly distributed over the coil, in order that a more uniform tube wall temperature is obtained. This causes the fuel consumption to decrease. Also, it will be possible for the length of each coil to be adapted more closely to the desired conversions, which results in optimum minimum dimensions of the radiant section of the furnace.

A simple embodiment of a furnace according to the invention is that in which each coil consists of three tubes. The coils, of which there should be present two or more in an even number, may all be located in one level plane, which plane is located between and at equal distances from the aforementioned vertical walls of the chamber. The burners then may have the same capacity and be arranged in pairs, the two burners of one pair being arranged symmetrically relative to the set of adjacent inlet openings of two coils. This results in a flat, vertical furnace with a recurrent pattern of burners and coils comprising two burners and two coils.

Another embodiment is obtained when the coils are arranged in two or more level planes that are parallel to each other and to the aforementioned vertical walls of the chamber, and in such a way that in each plane the same-evennumber of banks are present. The burners then may be arranged in one or more groups of at least three, so that the burners of one group are located on a line that is perpendicular to the level planes of the coils located in one plane, in such a way that always one bumer is present between a vertical wall and the nearby inlet openings and one burner is present between two planes containing coils, the burners present between the planes containing coils having a larger capacity than the burners present near a vertical wall. The ratio between the capacities may be approximately 2: l. A furnace having this arrangement is less flat than the one described hereinbefore. With this arrangement, to obtain a given capacity, a smaller wall area of the chamber is required, which is favorable in view of the walls being very costly owing to the high requirements concerning heat resistance. A furnace of this type is particularly suitable for applications where large capacities are required.

The burners should give only short, vertical flames, that is short relative to the length of a tube, the flame length being, for instance, at most one-third of the tube length. In this way it is ensured that the heat production does not take place over the entire length of the nearby tubes but remains limited to the neighborhood of the inlet openings. By using burners capable of producing a flame of low luminosity, the requirement of an uneven distribution of the heat production over the length of the pipe is met even more fully. Particularly suited are burners of the multiple vortex type. With these burners the combustion takes place mainly in a combustion chamber which is integrated with the burner into a combustor. This combustion chamber communicates with the chamber of the furnace that contains the tubes. The burners produce a flow of combustion gas of a high temperature. These burners can be fired with fuel of almost any type. It is preferred that the burners be present in the bottom part of the chamber. The stream of hot gas then enters the chamber in the vertical, upward direction. Along the vertical, in the upward direction, the temperature of the gas decreases. The discharge of combustion gases takes place in the upper part of the chamber via one or more openings.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a vertical cross-sectional view of a furnace according to the invention.

FIG. 2 is a cross-sectional view of the furnace of FIG. 1 taken along the lines [1-1] of FIG. 1.

FIGS. 3 and 4 show cross-sectional views similar to the view of FIG. 2 of two embodiments of furnaces according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, the vertical sidewalls l and la of the chamber 2 of a furnace 2a are shown. In chamber 2, which represents the radiant section of the furnace 2a, are located, in the bottom thereof, burners 3, 4, 5 and 6 (FIG. 2). The combustion gases are discharged via an opening 7 (FIG. 1) near the roof lb of the chamber 2. The medium to be heated is passed through a line 8 to a heat exchanger 9. In this heat exchanger 9 the medium is preheated by the combustion gases discharged via opening 7. In heat exchanger 9, the heat transfer takes place mainly by convection, for which purpose special constructions are known in the art. The heat exchanger 9 may be present on the roof lb of chamber 2, but may also be placed adjacent that chamber 2. Chamber 2 contains a number of vertical tubes which are combined into coils in such a way that each coil contains an odd number of tubes and that an even number of coils are present. For example, in FIG. 1, four coils are present, indicated by reference numbers I0, 11, '12 and 13. Each coil here consists of three tubes. At the underside of the furnace 1a are present the connections to line 14 for the introduction of the medium; at the upper side are present the connections to line for the discharge of the product obtained by heating the medium.

The coils 10 through 13 are preferably located in one plane and are so arranged that each coil is the mirror image of the adjacent one. As a result, the tubes 10a and lla are adjacent, and so are the tubes 12a and 130. It is through these tubes that the medium to be thermally treated enters. Next to these tubes, at the beginning thereof, are located the burners 3, 4, 5 and 6 in the manner indicated. These burners are mounted vertically and preferably produce nonluminous, short flames.

The tubes 10a, lla, 12a and 13a receive-owing to the vertical, upward flows of hot combustion gas-more heat than the other tubes of each coil, which is favorable, since the medium should absorb the highest heat flux immediately after entrance. At least wall I and la are relatively smooth, vertical and parallel to each other.

The reference numerals used in FIG. 2 have like meaning as in FIG. 1. FIG. 2 clearly shows the symmetrical arrangement of the burners 3 through 6 relative to the tubes 10a through 13a with inlet openings. The tubes through which the medium flows in the upward direction are indicated by cross sections with a dot in the center, the tubes through which the medium flows in the downward direction by cross sections containing a cross.

A cross section of a furnace according to the invention where the coils are grouped in two planes is shown in FIG. 3. The coils again consist of three tubes each. The coils l6, l7, l8 and 19 are located in one level plane; the coils 20, 21, 22 and 23 in another level plane, parallel to the first. Here again, the coils are arranged in such a way that each coil is the mirror image of the adjacent one. There are two groups of three burners. The burners 24, 26 and 26 are in a line between the coils l6 and 17,20 and 21. The burners 27, 28 and 29 are arranged similarly relative to the coils l8 and 19, 22 and 23. The burners 25 and 28 have a larger capacity than the other burners, such as a ratio of approximately 2:l. A furnace according to FIG. 3 has a large capacity within a comparatively small wall area of the chamber.

In the embodiment according to FIG. 4 four coils30, 31, 32 and 33are located in one plane. For each coil holds that the distance between the first tube and the second, counted from the tube with the inlet opening, is larger than the distance between the second and the third tube. The burners 34 and 35 belong to the coils 30 and 31, the burners 36 and 37 to the coils 32 and 33. The inequality of the distances between the tubes of one coil makes it possible to obtain a certain uneveness of the net heat flux to those tubes, as a result of which the condition of a uniform tube wall temperature is approximated more closely.

I claim as my invention:

1. A cracking furnace for carrying out chemical reactions therein, said furnace comprising:

a closed chamber having a top wall, a bottom wall and a plurality of sidewalls, at least two of said sidewalls being relatively smooth, vertical and parallel to one another;

a plurality of vertically extending hollow tubes for carrying a medium therethrough being disposed within said chamber;

an odd number of said tubes being combined into a coil, all of said tubes within one coil being joined to adjacent tubes within the same coil in such a manner that connections at the bottom end of each adjacent tube alternates with connections at the top end of each adjacent tube, an even number of said coils being disposed within said chamber;

each of said coils including inlet and outlet openings, all of said inlet openings being disposed on the same side of said chamber and all of said outlet openings being disposed on the side of said chamber opposite the inlet openings;

at least two of said coils being disposed adjacent each other in such a manner that one of the coils is the mirror image of the other, all of said tubes therein being located in common plane in such a manner that the inlet openings of at least two adjacent coils are adjacent one another;

burner means operatively engaging said inlet openings for providing flames to said inlet openings; and

at least one combustion gas outlet disposed in said chamber on the side of said chamber communicating with the outlet openings of said coils.

2. The furnace of claim I wherein the distances between adjacent tubes of one coil increases along the route from the tube with the outlet opening, therein to thetube with the inlet opening therein.

3. The furnace of claim 1 wherein each coil consists of three tubes.

4. The furnace of claim 1 wherein all of said coils are located in one level plane, which plane is located between and at equal distances from said vertical sidewalls of the chamber.

5. The furnace of claim 1 wherein said burner means includes a burner associated with each inlet opening, each burner having the same capacity and being arranged in pairs, the two burners of one pair being arranged symmetrically relative to the set of adjacent inlet openings of two adjacent coils.

6. The furnace of claim I wherein the coils are arranged in at least two level planes that are parallel to each other and to said vertical sidewalls of the chamber, in such a way that in each plane the same even number of coils are present.

7. The furnace of claim 6 wherein said burner means includes a burner associated with each inlet opening, each burner being arranged in at least one group of at least three burners so that the burners of on group are located on a line that is perpendicular to the level planes of the coils and that runs symmetrically between the adjacent inlet openings of two coils located in one plane, in such a way that one burner is al- 5 jacent a vertical sidewall.

8. The furnace of claim 7 wherein the ratio between the capacities of said burners present between said planes to the other of said burners is approximately 2: l.

9. The furnace of claim I wherein said burner means produce flames of relatively low luminosity.

10. The furnace of claim 9 wherein said burner means are of the multiple vortex type.

11. The apparatus of claim 1 wherein the burner means is located in relation to at least two of said coils so that the second and following tubes of each coil, counting from the tube with the inlet opening, are farther removed from said burner means.

12. The apparatus of claim 1 including preheater means adapted to preheat the medium with combustion gas from the furnace chamber. 

2. The furnace of claim 1 wherein the distances between adjacent tubes of one coil increases along the route from the tube with the outlet opening, therein to the tube with the inlet opening therein.
 3. The furnace of claim 1 wherein each coil consists of three tubes.
 4. The furnace of claim 1 wherein all of said coils are located in one level plane, which plane is located between and at equal distances from said vertical sidewalls of the chamber.
 5. The furnace of claim 1 wherein said burner means includes a burner associated with each inlet opening, each burner having the same capacity and being arranged in pairs, the two burners of one pair being arranged symmetrically relative to the set of adjacent inlet openings of two adjacent coils.
 6. The furnace of claim 1 wherein the coils are arranged in at least two level planes that are parallel to each other and to said vertical sidewalls of the chamber, in such a way that in each plane the same even number of coils are present.
 7. The furnace of claim 6 wherein said burner means includes a burner associated with each inlet opening, each burner being arranged in at least one group of at least three burners so that the burners of one group are located on a line that is perpendicular to the level planes of the coils and that runs symmetrically between the adjacent inlet openings of two coils located in one plane, in such a way that one burner is always present between a vertical sidewall and the nearby inlet openings and one burner is present between two planes containing coils, the burners present between the planes containing coils having a larger capacity than the burners present adjacent a vertical sidewall.
 8. The furnace of claim 7 wherein the ratio between the capacities of said burners present between said planes to the other of said burners is approximately 2:1.
 9. The furnace of claim 1 wherein said burner means produce flames of relatively low luminosity.
 10. The furnace of claim 9 wherein said burner means are of the multiple vortex type.
 11. The apparatus of claim 1 wherein the burner means is located in relation to at least two of said coils so that the second and following tubes of each coil, counting from the tube with the inlet opening, are farther removed from said burner means.
 12. The apparatus of claim 1 including preheater means adapted to preheat the medium with combustion gas from the furnace chamber. 